KFC has been forced to close a raft of stores after a new delivery contract with DHL resulted in chicken shortages across the country.
The company said that of those outlets that were still open, many were offering a limited menu or had cut their hours.
It gave no notice of when operations would be back to normal.
"We've brought a new delivery partner onboard, but they've had a couple of teething problems - getting fresh chicken out to 900 restaurants across the country is pretty complex!" KFC said in a statement.
"We won't compromise on quality, so no deliveries has meant some of our restaurants are closed and others are operating a limited menu, or shortened hours," the company added.
The Colonel is working on it. pic.twitter.com/VvvnDLvlyq
-- KFC UK & Ireland (@KFC_UKI) February 17, 2018
"We know that this might have inconvenienced some of you over the last few days, and disappointed you when you wanted your fried chicken fix - we're really sorry about that."
KFC was previously a client of Bidvest Logistics but has since swapped contract to enlist DHL for food deliveries.
The chicken chain gave a "shout out" to its restaurant teams, who the company said were "working flat out to get us back up and running again".
In its own statement, DHL said: "Due to operational issues a number of deliveries in recent days have been incomplete or delayed.
"We are working with KFC and our partners to rectify the situation as a priority and apologise for any inconvenience this may have caused."
DHL announced in November that it had been appointed alongside QSL to manage the supply and distribution of food products and packaging for more than 850 KFC restaurants throughout the UK.
It said DHL would manage the physical warehouse and distribution service, with KFC adding it had specifically chosen the pair for their reputation of "innovation in logistics" across other industries.